The new material (\mathrm{Ba}{5}\left(\mathrm{~B}{2} \mathrm{O}{5}\right){2} \mathrm{~F}_{2}) represents the first confirmed structure of a pyroborate-halide anionic mixture in a crystalline phase. The structure contains four formula units in the monoclinic space group (C 2 / c) (No. 15) with cell parameters (a=20.726(3) \AA, b=7.115(2), c=8.589(2), \beta=95.05(5)^{\circ}), and (V=1261.7(5) \dot{A}^{3}). Ws characterization reveals a three-dimensional structure of three face- and edgesharing Ba-centered polyhedra that are also united through nonplanar pyroborate anions. These irregular polyhedra are eight- and nine-coordinate, and contain one or two (\mathrm{F}) atom(s) at their peripheries. The geometry of the pyroborate is unusual in that the principal planes of the double triangles are rotated from one another by (86.6(1)^{\circ}), the closest angular approach to the theoretical (90^{\circ}) limit of any known pyroborate; its B-O-B angle of (121.6(5)^{\circ}), however, is typical. Differential thermal analysis reveats single events on heating and cooling that are indicative of a congruently melting compound. (1993 Academic Press, Inc.